<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a = document.getElementById("txt1").value;
//获取第一个输入框的值
var b = document.getElementById("txt2").value;
//获取第二个输入框的值
var c = document.getElementById("select").value;
//获取选择框的值
//获取通过下拉框来选择的值来改变加减乘除的运算法则
var sum;
switch(c){
case "+": sum=parseInt(a) + parseInt(b);break;
case "-": sum=parseInt(a) - parseInt(b);break;
case "*": sum=parseInt(a) * parseInt(b);break;
case "/": sum=parseInt(a) / parseInt(b);break;
}
//设置结果输入框的值
document.getElementById("result").value=sum;
}
</script>
</head>
<body>
<input type='text' id='
